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Lesson 1

Infrequent Recovery Situations

Imagine a situation like this: Before you manage to complete an open database backup, your system crashes and brings down the database. You cannot open the database because the datafile in the hot backup mode is not synchronized with the database. The following questions must be answered before you proceed.
  1. Does this datafile need a recovery?
  2. What can you do?
Learning how to solve problems like the one above is the focus of this module. You have already learned how to perform complete and incomplete recovery to an ARCHIVELOG database in various situations. There are, however, other occasions when a database or datafiles may need some special attention. Although these situations do not occur frequently, as a DBA you should be prepared for these events.
This module investigates recovery situations that can happen on occasion and provides possible solutions to handle these problems. In addition, the maintenance of online redo logs is covered.
It is important to remember that all topics in this module are for a database running in ARCHIVELOG mode.

Redo Log Maintenance serves as a Pivotal Component

In Oracle database systems operating in ARCHIVELOG mode, Redo Log Maintenance serves as a pivotal component in ensuring data integrity, availability, and recoverability. The fundamental purpose of maintaining redo logs is to capture all changes made to the database, thereby providing a robust framework for recovery operations, data auditing, and optimal system performance. Understanding the nuances of Redo Log Maintenance is essential for an Oracle Database Administrator (DBA) tasked with maximizing uptime and minimizing data loss in enterprise environments.

Key Objectives of Redo Log Maintenance:

  1. Database Recovery: The most critical function of redo logs is to facilitate database recovery. In the event of system failures, data corruption, or other catastrophic incidents, the redo logs serve as an invaluable resource for rolling forward committed transactions and restoring the database to a consistent state.
  2. High Availability: Redo logs contribute to high availability by allowing seamless switching and archiving of log files without disrupting database operations. This ensures that the database can continue to function efficiently even during backup operations or maintenance tasks.
  3. Data Integrity: By persistently storing every modification made to the database, redo logs act as a safeguard against data corruption. They serve as an audit trail that can be analyzed for inconsistencies or unauthorized data changes.
  4. Performance Optimization: Proper management of redo logs enables performance tuning. Factors such as log size, number of log files, and disk placement can be optimized to reduce I/O contention and thereby enhance database performance.
  5. Standby and Replication Support: Redo logs play an essential role in Data Guard configurations and other standby or replication strategies. They enable real-time data shipping and synchronization between primary and standby databases, ensuring data coherency and disaster recovery capabilities.
  6. Point-in-Time Recovery: In ARCHIVELOG mode, redo logs can be employed for more granular recovery options, such as point-in-time recovery or tablespace-level recovery, offering greater flexibility and control during restoration tasks.

Key Maintenance Tasks:

  1. Log Switching and Archiving: Regular log switches need to be monitored to ensure that the redo logs are archived properly. Insufficient archiving can lead to system stalls and impact availability.
  2. Log Sequence Number Management: DBAs must closely monitor and possibly reset the log sequence numbers to avoid reaching the maximum limit, which could otherwise impede database operations.
  3. Storage Management: Allocating appropriate disk resources and managing the archival destinations are crucial tasks. Running out of disk space for redo logs or archive logs can halt database operations.
  4. Monitoring and Alerting: Implement comprehensive monitoring strategies to receive real-time alerts on log-related issues such as space constraints, errors during log switches, or archive failures, to take timely corrective actions.
  5. Purging and Deletion: Old archived logs should be safely purged once they are no longer needed for recovery, replication, or auditing purposes. This helps in optimal utilization of storage resources.

In summary, Redo Log Maintenance in Oracle databases running in ARCHIVELOG mode is not a mere operational afterthought; it is a cornerstone of robust database architecture. The overarching purpose is to ensure an impregnable recovery strategy, facilitate high availability, and maintain peak performance, all while preserving the sanctity and integrity of the data. Failing to meticulously maintain redo logs could compromise these critical facets, leading to catastrophic outcomes that could jeopardize the entire data management ecosystem.


Put the Database in ARCHIVELOG Mode

You do not need to shut down the database and then restart it to put it in ARCHIVELOG mode. You can put the database in ARCHIVELOG mode while it is running. To do this, you use the ALTER DATABASE ARCHIVELOG command. Here are the steps on how to put an Oracle database in ARCHIVELOG mode:
  1. Connect to the database as a user with SYSDBA privileges.
  2. Issue the following command: 
    ALTER DATABASE ARCHIVELOG;
  3. Respond to the prompt with the following: 
    YES

The database will now be in ARCHIVELOG mode. All redo log files generated by the database will be archived to a disk or other storage location. Here are some of the benefits of putting an Oracle database in ARCHIVELOG mode:
  • Point-in-time recovery: You can use archived redo log files to recover the database to any point in time up to the last archived redo log file.
  • Media recovery: If you lose the data files for a database, you can use archived redo log files to recover the data files.
  • Standby databases: You can use archived redo log files to create a standby database, which is a copy of a production database that can be used for disaster recovery.

Module Objectives

After completing this module, you will be able to:
  1. Recover a datafile with no backup
  2. Recover a file in backup mode
  3. Clear a corrupted online redo log file
  4. Perform a recovery with inactive online redo logs
  5. Recreate online redo log files
  6. Obtain recovery status information
The next lesson demonstrates how to recover a lost datafile with no backup.